1. Field of the Invention
This invention relates to controlling apparatus used for heating water. More particularly, the invention relates to a combination of a heat pump water heater with a separate water storage tank including apparatus and a method for controlling operation of the heat pump water heater in response to the temperature maintained within the storage tank.
2. Prior Art
Heat pump water heaters are known in the art. A heat pump water heater is typically an add-on unit used in addition to an existing electric or oil fired hot water heating unit. The heat pump water heater typically includes a refrigeration circuit having a compressor, condenser, evaporator and expansion device connected to form a refrigeration circuit. The refrigeration circuit is used to transfer heat energy from the air in heat exchange relation with the evaporator to water to be heated in heat exchange relation with the condenser. Water is typically circulated between the hot water tank and the heat pump unit such that a reservoir of heated water is maintained in the tank.
One of the problems of supplying add-on heat pump water heaters is to integrate the controls such that appropriate water temperature levels are maintained within the hot water tank. Often, a separate heating means such as electric resistance heaters or an oil burner for heating the water in the hot water tank is de-energized upon the installation of a heat pump water heater. The purpose of installing a heat pump water heater is to save energy since the amount of electricity required to transfer heat energy using a heat pump is less than the amount of electricity required to heat the water by electric resistance heat or the equivalent amount of oil required for oil heat. Of course, heat pump hot water heaters can be utilized with tanks having no alternate heating source or with gas or propane fired hot water heaters.
One of the problems associated with an add-on unit is that water connections must be made between the hot water tank and the heat pump water heater unit. Additionally, the temperature of the water to be delivered to the enclosure for domestic or other purposes must be maintained at the desired level. In a typical hot water heater a thermal sensing device is integrated with the water heater controls to regulate the electric resistance element or oil burner. This sensing element is usually mechanical in nature and is not suitable for use with an add-on heat pump water heater.
Several methods of integrating the control of a heat pump water heater unit with a hot water tank have been utilized. One method is to mount a temperature sensor on the external wall of the hot water tank. This temperature sensor then detects a temperature which is indicative of the water temperature within the tank. However, the amount of insulation between the tank and the exterior surface to which the sensor is mounted, the location of the sensor on the tank and the heat exchange between the area adjacent to the sensor and the surrounding ambient air all effect the temperature detected. Additionally, Underwriter Labratories, Inc. and certain local code requirements prohibit electrical connections being made to the surface of the tank.
Another method utilized is to continuously operate a circulating pump circulating water from the tank to the heat pump water heater unit. This method allows the water to be constantly monitored by measuring its temperature somewhere along the water flow path. The disadvantage of this type of control is that the pump runs continuously drawing electrical power to circulate the water. Additionally, the temperature stratification desired in the tank to keep the hot water at the top and cold water at the bottom is impaired by the constant circulation of water.
The apparatus and method outlined herein include connecting a thermally conducting tubular member to the tank and mounting a thermal sensing element thereto. The entire assembly is insulated to reduce heat transfer between the tubular member and the thermostat and the ambient air. The thermostat is connected to energize the circulating pump and the refrigeration circuit of the heat pump water heater only when the appropriate temperature conditions are sensed. Hence, the energy utilized to constantly operate the pump motor is reduced to that necessary to operate the circulating pump only when the refrigeration circuit is energized. Additionally, the stratification of the water in the hot water tank is not reduced by continually operating the circulating pump. By providing a thermally conductive member and by insulating that member a temperature is sensed which is representative of the temperature in the tank at that particular point in time.